1. Field of the Invention
The present invention relates to an image encoding apparatus that generates quantized orthogonal transform coefficients of an image, and encodes the image by grouping together quantized coefficients of a predetermined size.
2. Description of the Related Art
Well known conventional image encoding apparatuses encode an image by grouping together quantized coefficients, of a predetermined size, obtained by orthogonal transformation and quantization of the image.
As the image input devices such as digital cameras are getting more advanced with larger number of pixels, the compressed Joint Photographic Experts Group (JPEG) data size is getting larger. Hence, there is an increasing need to reduce the JPEG data size so that the image data size reduces or more data can be stored in the same storage capacity.
In the JPEG image compression technique, changing the quantization threshold generally lowers the encoding bit number. Specifically, increasing the quantization threshold increases the invalid coefficients, and reduces the valid coefficients, thereby resulting in a low encoding bit number.
However, each image has only one type of coefficients. Therefore, changing the quantization threshold also changes the value of all the coefficients having frequency component that is same as the changed quantized coefficient, thereby adversely affecting the image quality.
Japanese Patent Laid-Open Publication No. H6-70174 and Japanese Patent Laid-Open Publication No. H10-200892 disclose a method for reducing the encoding bit number and suppressing its effect on the image, by partially removing valid coefficients. Specifically, to lower the encoding bit number, the valid coefficients are reduced by assigning the value of zero to some of the valid coefficients of quantized Discrete Cosine Transformed (DCT) coefficients.
Although in the above conventional technology, changing the quantization threshold prevents image degradation, the image quality is still adversely affected to a certain degree, because the value of zero is assigned to some valid coefficients.
Additionally, in the aforementioned conventional technology, to prevent an adverse effect on the image, absolute values can be assigned only to a small number of valid coefficients. Thus, the number of valid coefficients are limited, which itself acts as a limiting factor for lowering the encoding bit number.